1. Field of the Invention
The present invention relates to an insulating paste composition and a conductive paste composition for a circuit board.
2. Related Art
A thick film circuit board is known as a circuit board, which comprises an insulating substrate formed by, for example, alumina, having thereon a conductive paste composition printed and baked. The conductive paste composition comprises conductive metal powder main components of which are silver and palladium (Pd) or a noble metal such as platinum, and an inorganic binder such as a glass frit, which are dispersed in an organic vehicle.
Recently, a circuit board is highly demanded to have a high density, a compact size and environment durability. For example, in an IC circuit board for an automobile, it is considered that high integration due to the use of hybrid ICs is demanded, and the installation in an engine room of the board, which has been installed in a vehicle cabin, is required.
Accordingly, wiring parts formed on an insulating substrate are required to have higher reliability characteristics than the past. For example, even when a voltage is applied between the wiring parts having a narrow wiring distance therebetween and the board is put under an outer environment for a long period of time, the insulating property between the wiring parts should not be deteriorated.
However, it is generally known that, when the wiring part is formed by using a conductive paste containing silver on an insulating substrate, silver contained in the wiring part is ionized on application of a voltage in the presence of water or water vapor and moves from a positive electrode part to a negative electrode part causing dendrites of silver, and the so-called migration of silver is liable to occur, which results in deterioration in insulating property between the wiring parts. Therefore, a sealing structure has been employed to prevent the migration of silver.
However, the inventors found a phenomenon in that the insulation resistance is lessened on application of a voltage under a high temperature atmosphere even if water or water vapor is not present. The phenomenon is described below with reference to the results of experiments conducted by the inventors.
The phenomenon can be summarized as follows. For example, when wiring parts containing silver are arranged on an insulating substrate with a narrow distance of about 100 .mu.m therebetween, a voltage of about 16 V (the maximum standard for electric devices installed in an automobile) is applied between the wiring parts, and it is put under a high temperature atmosphere of about 150.degree. C., the insulating resistance between the wiring parts is greatly lowered with the lapse of time, and at last, insulation failure therebetween occurs.
The phenomenon occurs under an atmosphere in that no water content is present, such as at a temperature of 100.degree. C. or higher, or in vacuum. Furthermore, when the wiring parts are in a state of substantially short-circuit, dendrites of silver is not observed between the wiring parts, which must be observed on generation of the migration under the presence of water content, and polarity is also not observed in the phenomenon (i.e., the phenomenon occurs irrespective of the polarity of a positive electrode part and a negative electrode part). Accordingly, it can be determined that this is a phenomenon newly found, which is different from the migration occurring under the presence of water content. Such a phenomenon is called a high temperature leakage phenomenon hereinafter.
Although the mechanism of the phenomenon is not completely clarified, the summary is as follows according to the investigation by the inventors.
Conventionally, wiring parts on a general thick film circuit board are obtained by printing a conductive paste composition on an insulating substrate such as a ceramic substrate formed by alumina, followed by baking. The conductive paste composition comprises conductive metallic powder containing silver and an inorganic binder such as glass frit, which are dispersed in an organic vehicle. The conductive paste composition is generally baked at a temperature of from 800.degree. C. to 900.degree. C. During the baking, parts of the silver and the inorganic binder are evaporated from the wiring parts formed on the insulating substrate by printing, and they are attached as a reaction product containing silver on the insulating substrate. Since the reaction product is generally present in an extremely thin form, it is generally not observed. The reaction product has an insulating property, which is the same as the underlying insulating substrate.
However, when a voltage is applied between the wiring parts between which the reaction product is present and it is put under a high temperature atmosphere without influence of water content (which is called a high temperature atmosphere hereinafter), silver contained in the reaction product comes to have conductive nature, and the insulating property between the wiring parts is lessened with the lapse of time.
The mechanism of the high temperature leakage phenomenon summarized above was clarified by the following some verification experiments (verification experiment A and verification experiment B).
The verification experiment A is described below. A circuit board was prepared, on which wiring parts having plural wiring distances different from each other were formed by using a general conductive paste composition containing silver by printing and baking on a ceramic substrate formed by alumina. A voltage was applied between the wiring parts, and it was put under a high temperature atmosphere. In the verification experiment A, the voltage applied between the wiring parts had a value (for example 60 V), which was higher than the voltage generally applied, for reaction enhancement.
It was found as shown in FIG. 1 that the insulating resistance value between the wiring parts was lowered with the lapse of time under the high temperature atmosphere. FIG. 1 is a graph showing the relationship between the time lapsing under a high temperature atmosphere and the insulating resistance value (Log10X, unit:.OMEGA.). In the plot marks in the figure, white circles and black circles are for the case of the temperature 125.degree. C., white triangles and black triangles are for the case of the temperature 150.degree. C., white circles and white triangles are for the case of a wiring distance 100 .mu.m, and black circles and black triangles are for the case of a wiring distance 50 .mu.m.
At these cases, appearance change such as the known dendrites of silver generated by water content was not observed at the wiring parts between which insulating resistance value lowers. Therefore, it was found that the phenomenon had nature influenced by conditions, such as the wiring distance, the applied voltage and the ambient temperature.
Furthermore, the time when the insulating resistance value lowers to 100 M.OMEGA. was investigated as an insulation failure time in the relationship with the electric field intensity, which could be obtained from the voltage applied between the wiring parts. The results are shown in FIG. 2. FIG. 2 is a graph showing the relationship between the electric filed intensity and the insulation failure time. In the plot marks in the figure, circles are for the case of a temperature 150.degree. C., and triangles are for the case of a temperature 125.degree. C.
It was found from the results of the verification experiment A shown in FIGS. 1 and 2 that the deterioration of insulating property between the wiring parts depends on the wiring distance and the electric field intensity determined by the applied voltage, and is farther accelerated, the higher the ambient temperature is.
The detail observation was carried out to investigate the reason of the deterioration of insulating property.
No appearance change was observed before and after the insulation deterioration by the observation using a scanning electron microscope (SEM) with respect to the wiring parts, between which the insulation deterioration was observed, and its surroundings. Furthermore, the substances present in the same place were analyzed by an electron probe microanalyzer (EPMA) equipped with a wavelength dispersing X-ray analyzer, but no difference was observed before and after the insulation deterioration.
However, parts of the silver and the inorganic binder as the compositions of the conductive paste, which were not contained in the ceramic substrate by itself, were observed on the ceramic substrate after the insulation deterioration. They were present in the surroundings of the wiring parts, and the larger their amount, the nearer to the wiring parts.
The specific state where they were present was observed by a transmission electron microscope (TEM). As a result, it was confirmed as shown in FIG. 3 that fine silver particles A3 are present in the part of the glass substance A2 present at the boundaries of the alumina particles A1.
The reason of the above is estimated as follows. In the surroundings of the wiring parts which is formed by printing and baking the conductive paste, parts of the silver and the inorganic binder are evaporated and attached on the ceramic substrate when the conductive paste is baked. At this time, the glass substance present at the boundaries of the alumina particles of the alumina ceramic substrate is softened, and the evaporated material is liable to be absorbed.
It is of course that when a ceramic substrate, on which no conductive paste is printed and baked, is observed in the similar manner, the attachment of the compositions of the conductive paste, such as silver particles, is not observed. It was thus found that parts of the silver and the inorganic binder contained in the conductive paste contributes to the high temperature leakage phenomenon in any way.
The following verification experiment (verification experiment B) was conducted to investigate the reason why parts of the silver and the inorganic binder are present in the surroundings of the wiring parts. As a result, it was confirmed that a substance produced by reacting the inorganic binder and the silver (abbreviated as reaction product hereinafter) is attached onto the substrate when the conductive paste is baked, and electrical conduction thereof is caused by the silver contained in the reaction product.
As a first step of the verification experiment B, the verification experiment B1 using a board .alpha., a board .beta. and a board .gamma. is described.
The board .alpha. is a general circuit board, in which the deterioration of an insulation property occurs in the wiring parts. The board .beta. was produced by the following manner. Wiring parts were formed by printing and baking a conductive paste containing silver on an insulating substrate, and a groove was formed on the surface of the insulating substrate between the wiring parts in parallel to the wiring parts by a trimming method using laser, which is generally used to adjust resistance of a thick film resistor (abbreviated as the laser trimming hereinafter), so as to mechanically remove the reaction product present between the wiring parts.
The board .gamma. was produced by firstly forming a groove on the surface of the insulating substrate between the wiring parts in the same manner as described above using the laser trimming, and then forming the wiring parts by printing and baking.
A voltage was applied between the wiring parts of each of the board .alpha., the board .beta. and the board .gamma., and they were put under a high temperature atmosphere. As a result, it was found that the insulating property was deteriorated only in the board .alpha. and the board .gamma..
It is understood from the above that in the surroundings of wiring parts formed by printing and baking a conductive paste containing silver on an insulating substrate, part of the conductive paste composition is attached on the insulating substrate to deteriorate the insulating property between the wiring parts under the certain conditions. That is, it was understood from the verification experiment B1 that part of the conductive paste composition is attached on the insulating substrate by baking.
As the second step of the verification experiment B, the verification experiment B2 using a board .delta. and a board .epsilon. is described below with reference to FIGS. 4 to 6.
The board .delta. was formed by an insulating substrate b1 and a thick conductive paste a1 printed on the substantially whole surface of the substrate b1, as shown in FIG. 4. The board .epsilon. was formed by an insulating substrate f1 and gold wiring parts e1 formed thereon by using a gold material, which was confirmed not to cause insulation deterioration, as shown in FIG. 5.
The board .delta. and the board .epsilon. were arranged in such a manner that they faced to each other with a spacer c1 interposed therebetween to form an arbitrary gap d1, as shown in FIG. 6, and they were baked at a temperature of baking the conductive paste a1. A surface h of the insulating substrate f1 between the gold wiring parts e1 of the board E was observed with an EPMA apparatus, and it was confirmed that parts of the silver and the inorganic binder, which were the compositions of the conductive paste a1, were present on the surface h1.
The distribution of the silver and the inorganic binder was the substantially the same as the conductive paste a1 printed on the insulating substrate b1, i.e., the whole surface of the board .delta.. A voltage was applied between the gold wiring parts e1 in such a state, and it was confirmed that the insulating property between the gold wiring parts e1 was lowered under a high temperature atmosphere.
Accordingly, it can be said that parts of the silver and the inorganic binder are evaporated and scattered from the general conductive paste containing silver during the baking, i.e., in the verification experiment B2, they are scattered from the board .delta. to the board .epsilon. through the gap d1, and they are attached on the surface of the surrounding insulating substrate as a reaction product. That is, the route of attachment of the parts of the silver and the inorganic binder during the baking of the conductive paste was found from the verification experiment B2.
As the third step of the verification experiment B, the verification experiment B3 using the board .delta. and the board .epsilon. is described with reference to FIGS. 7A and 7B.
The board .delta. and the board .epsilon. were arranged in such a manner that they faced to each other with a spacer c2 to form an arbitrary gap g1, as shown in FIG. 7A. Further, the board .delta. and the board .epsilon. were arranged in such a manner that they faced to each other with a spacer c3 to form an arbitrary gap g2 shorter than the gap g1, as shown in FIG. 7B. As a result, the scattering distance of the reaction product could be changed between the devices shown in FIGS. 7A and 7B, and the amount of the reaction product attached on the board .epsilon. could also be changed therebetween.
That is, by changing the distance g1, g2 between the boards .delta., .epsilon., the amount of the silver attached on the board .epsilon. could be changed. The larger the distance g between the boards .delta., .epsilon., was, the smaller the attached amount of silver was. A voltage was applied between the wiring parts e1 of each of the devices shown in FIGS. 7A and 7B under a high temperature atmosphere. As a result, it was found that difference in insulation deterioration occurred depending on the attached amount of silver, as shown in FIG. 8. FIG. 8 is a graph showing the relationship between the attached amount of silver and the insulation failure time.
It can be said from the verification experiment B3 that occurrence of the phenomenon depends on the amount of silver existing as the reaction product on the surroundings of the wiring parts, and when the attached amount of silver is larger, the insulation deterioration is promoted.
Therefore, it was confirmed from the verification experiment B that the reaction product present on the surroundings of the wiring parts is scattered in the air during baking of the conductive paste and attached onto the insulating substrate, and the insulation deterioration is caused by the silver contained in the reaction product, which becomes conductive on application of a voltage under a high temperature atmosphere.
The high temperature leakage phenomenon would not be found because of the following reasons. In the conventional circuit board, a sealing structure was employed to prevent the migration of silver under an atmosphere containing water or water vapor. The reaction product causing the high temperature leakage phenomenon is normally not observed, and the appearance of the board is not changed even when silver of the reaction product becomes conductive to form a short circuit.
It can be said from the above that it is highly possible that the insulation failures that were considered to be caused by the migration of silver contain a certain number of cases due to the high temperature leakage phenomenon.
It is also considered from the above that because the substance (reaction product) to be a factor of the high temperature leakage phenomenon is formed and attached on the insulating substrate during the baking even under the conditions without water, the occurrence of the phenomenon in question is difficult to be prevented even by the sealing structure for preventing the migration of silver.